During the Upper Palaeozoic the development of several strike faults strongly influenced the deformation history of SW Wales. This aspect of the deformation is emphasized in this new description of the tectonic evolution of the region.
After the Caledonian orogeny, several strike faults caused crustal extension by normal dip-slip and partially controlled the subsidence of depositional basins. These faults ceased moving before the initiation of Variscan deformation in SW Wales. Two of the earlier faults were reactivated with reverse-slip translations during Variscan deformation, when three strike faults were initiated as a result of gravitational spreading. Two of the new faults formed sole thrusts for northward-translating nappes, and the other developed as a subsidiary lag fault. The sole thrusts transected some of the pre-existing faults, causing along-strike variation in their motion. The strike faults in SW Wales were therefore initiated at different times and display contrasting displacement histories.